My alternators on it's way out, when everythings on the voltage goes down to like 10.5V. It's fine however when the essential things like headlights, brake lights, heater fans and radio are on (around 13.5V) Just when you turn the front demisters on the voltage goes low. You can hear a slight whine

What is the cause of this? a diode gone perhaps? When I have rev's high (about 2,500rpm) the lights are less dim with the front demisters are on.

if you mean the heated window at the front its a mega high voltage item thats why its advised to be used only when necessary at idle the voltage will drop and the alt will whine if the voltage is okay with headlights and rear demist on and the voltage increases when you rev the car the alternators fine.

if you mean the heated window at the front its a mega high voltage item thats why its advised to be used only when necessary at idle the voltage will drop and the alt will whine if the voltage is okay with headlights and rear demist on and the voltage increases when you rev the car the alternators fine.

i was going to say if you needed a 105amp alternator i have a spare one which i could of let go for a small amount lol just collecting dust here.

even that checks by some garages cannot point out some faults that give the battery charge light. mine for example, i had the battery light constant. i went and checked the voltage on the battery was a bit low so i got the battery changed just incase a cell had died, light stayed on. went to a few garages/auto electricians they checked, scratched their heads and said could be the alternator so bought a new alternator and fitted it - light still on so the alternator was ok. checked the wiring as best as i could. turned out the ECU reference wire in the wheel arch was snapped. apparently this is really common on the focus's and transits.

you will find however, the heated windscreen wont work when this battery light is on. as this take's so much juice. it is also that taxing on the alternator/battery it is timed to auto turn off 10-15 mins. the back window heaters are timed but dont take as much power and so will stay activated.

with the engine running and a few revs given, everything on, the alternator should kick around 14.5v out. engine off, battery should read anywhere between 12.5v to 12v.

if your engine is running and everything is on, and your alternator is showing as kicking out 11.5v or lower, the alternator has gone into conventional mode and should show the light on the dash. as you say its kicking 10.5v...i would use a multimeter and get someone to put it to 2k revs and see if it increases correctly. if not i would suspect these wires and carry out checks to save you some £££'s of buying a new battery and or alternator first.